Field of the Invention
This invention relates generally to rubber compositions and more particularly to rubber compositions having components that increase the rigidity of the rubber composition.
Description of the Related Art
The manufacturing of certain rubber articles, including rubber components that may be found in tires, require a rubber composition that has a high rigidity. Tires and other articles that are made of rubber are manufactured from rubber compositions that include rubber, e.g., natural rubber, synthetic rubber or combinations thereof, as well as fillers, plasticizers, vulcanizing agents and other chemicals that improve the physical characteristics of the cured rubber composition.
Those skilled in the art understand that there are different ways of increasing a rubber composition's rigidity such as the addition of a reinforcing filler or a reinforcing resin. Reinforcing fillers are well known, examples of which include carbon black and silica. Reinforcing resins are also well known and typically include a methylene donor and a methylene acceptor.
In general, resins are typically (but not always) nonvolatile, solid organic substances that are produced naturally by plants or synthetically from petrochemicals or other sources of hydrocarbon materials. As used in rubber compositions, resins may be classified as either reinforcing resins or as plasticizing resins. Plasticizing resins are added to a rubber composition to improve the plasticity or workability of a rubber composition. They are often added as a substitute for or in addition to a processing oil and are known to improve the resulting physical characteristics of the cured rubber composition.
As noted, reinforcing resins are added to a rubber composition to increase the rigidity of the cured rubber composition. These reinforcing resins intermix with the rubber polymer chains and, when reacted with a linking agent or with each other, form a three-dimensional network that improves the physical characteristics of the cured rubber composition. Many of these resins are classified as being methylene acceptor/donor systems that react together to generate a three-dimensional reinforcing resin network by a condensation reaction.
As is often the case, whenever a rubber formulator attempts to improve a physical characteristic of a rubber composition, there is often a decrease or compromise resulting in another desired physical characterization of the rubber composition. For example by adding additional fillers or resins to a rubber composition that increases its rigidity there may be a compromise in its processability and/or mixing characteristics and/or in its cured cohesiveness characteristics. Therefore rubber formulators search for ways to increase a rubber composition's rigidity without a resulting compromise in other desired characteristics.